There is not much difference between:
- Él está de compras en una joyería.
- Él está comprando joyas en una joyería.
The tense in both sentences is the present indicative. While in (2) the gerund "comprando" forms a verb phrase (perífrasis verbal), in (1) we have a prepositional phrase formed by "de" and the noun "compras".
We can find other similar examples where the verb "estar" followed by a prepositional phrase is equivalent to "estar" + gerund:
- estar de paseo = estar paseando
- estar de viaje = estar viajando
- estar de visita = estar visitando
- estar de festejo = estar festejando
There might be a slight difference between the two: while "estar de + noun" may be used to mean that the person is engaged in an activity though not doing the action right now, "estar + gerund" may be used to mean that the action is currently in progress. Therefore, if we say:
it may be the case that he is at a shopping mall but having a coffee right now,
while if we say:
it may be the case that he is buying at the moment of speaking.