The 5050 is the smallest model in the Line (9090DB, 9090, 8080DB, 7070, 6060, 5050). Power rating is 30W.
If the one you mentioned is in nice shape, you can consider buying it, however, if its a thrift shop- my suggestion is to let the shop be the one to give you a really good deal on it at a price too good to pass up- unless you dont mind and just give him the price they want for it- its your money so its up to you to decide. Some people are very tight with their money and others a bit more carefree about it. If you go in there and look at the receiver for a while and then talk with the shop owner, and say things like you might need a receiver with some more power and youve seen some good deals on Ebay or wherever else Online- so on and so forth- you might be able to get the 5050 for a next-to-nothing price. My point is, being you're the customer, let the shop assume more of the role of trying to sell the receiver to you rather than you just going along with the price they want. Try and get an idea of how flexible they are on the price- based on your "concerns" about the amp's power, its age and the risk of possibly needing parts in the future for it and there arent any available nowadays"... Again- its up to you and your personality makeup- whether you are motivated to try and get the lowest price possible or whether you just want the receiver and would prefer to forego the haggling process and buy it for asking price.
I can tell you that, basically, the more power an amp/receiver has, and thus some extra features as well, then the pricetag will thus increase accordingly. The 5050 being the 'little guy' in the Line, is not going to be worth as much as much as the upper models in the Line. However, another way to look at it is that the smaller models are just that->smaller- and a positive aspect about that is, being smaller, its less complicated than the bigger models and most likely will be less prone to developing problems as there is less complex circuitry inside and all the parts and so forth are not so packed in together like the bigger models.
Personally, in that Line I think the 7070 is the 'best buy' in terms of what it offers- has the power meters and double the power of the 5050- and those are 60 VERY STRONG watts. They only cost a little more than a 5050 or 6060, but a bit less than a 9090. Avg price is about $150, give or take depending on condition.
And what I mentioned earlier about Ebay (you can buy from there instead of thrift shop, though shipping cost is extra $ unless seller happens to be in your local area)- well, that is quite true. There are no shortages of 9090's thru 5050's on there. Maybe some weeks there are less of them than others, but generally speaking, if you keep your eye on Ebay, you can see some potentially good buys on there. And thats why I wouldnt recommend putting much effort into getting that 8080DB you mentioned which doesnt sound like its in such hot shape. You'd be better off to spend a bit more and get one thats 100% on Ebay and has all the knobs etc.. Unless you want to get that 8080DB for a dirt cheap price and use it as a parts backup unit or you are inclined to disassemble it and sell it for Parts profit on Ebay.
Look through the Ebay listings and see which models interest you- do you like the 9090..5050 Line the best or maybe the G Series models appeal to you, or some other Line, or maybe an AU integrated- Sansui made smaller models like the AU-317...117 that would be excellent buys. The point is to buy something you really like and works well, rather than to just get whatever one the local thrift shop has.
Regards, B/F.